1. Field of the Invention
The present invention relates to methods and apparatus for separating materials of differing densities and, more particularly, relates to an improved system for efficiently removing oil or other hydrocarbons from waste water.
2. Description of the Background
Numerous devices have been constructed to remove oil or other contaminants from water. Most of these systems rely upon the difference in density between various liquids, and are thus theoritically capable of separating various solids and/or liquids from a different liquid. Since oil has a typical density of 0.346 lbs./per foot (columnar weight), and water has a density of 0.435 lbs./per foot, oil can be skimmed from the surface of water in a settling pond, while water flows under a weir to a discharge stream.
Various factors affect the manner in which petrochemical companies, manufacturing plants, municipal water departments, etc. are and will be handling the removal of oil from waste water. The quantity of water requiring removal of contaminants is tremendous, and billions of dollars are being expended annually to update existing waste water separation systems. Proponents often overrate the water flow volume which a particular separation system can efficiently handle, and users are justifiably skeptical of unproven claims. Environmentalists and government experts recognize that future regulations will control evaporation of hydrocarbons by requiring that settling tanks and ponds be enclosed, thereby substantially increasing the cost of those systems. While some experts tout biodegradation systems with "oil-eating bugs" as the key to future techniques for treating waste water, users recognize that such systems are both expensive and complicate downstream purification or separation systems.
U.S. Pat. No. 4,045,344 discloses a system for treating waste water including a bundle of submerged tubes with intermediate passageways. Organic substances are removed by decomposition caused by microorganisms or "bugs" which adhere to the tubes. U.S. Pat. No. 4,123,357 assigned to Chevron discloses a system for removing oil and deoiled solids from a sludge utilizing a combination of steps involving stirring, heating with a substantial input of mechanical energy, and sedimentation. U.S. Pat. No. 4,145,280 discloses a system for separating oil and water by passing the mixture first through a moving filter media, separating the coalesced substance by gravity, and filtering the remainder of the impurities from water. U.S. Pat. No. 4,356,086 discloses a vessel which floats on the surface of a body of water. Pumps are provided in the end of downwardly projecting tubes for pumping water in and out of the vessel. Oil is collected from the vessel, and may be subsequently pumped into a storage tanker.
U.S. Pat. No. 4,492,630 discloses a separator with a pair of tanks for separating hydrocarbons and water. The oil/water mixture is fed into the first tank, and water passes through a passageway connecting the first and second tanks. Oil is removed from the top portion of the second tank while water flows to an outlet provided in the bottom portion of the second tank. U.S. Pat. No. 4,597,863 utilizes many of the concepts disclosed in the '630 patent, and discloses a boat-like device for selectively propelling about the surface of a body of water to remove oil from the surface of the water.
One of the most commercially used oil/water separation systems is referred to as a "API" or "gun-barrel" system, which includes above-ground tanks typically each 50 to 100 feet in length. The oil/water mixture is continually input at one end of each tank, which is preferably sized to maintain a flow rate of about one knot. Oil is removed from the surface of the water at the other end of the tank, while water flows by gravity through a lower outlet to a discharge stream. The oil is commonly removed by a half-pipe located at the surface of the outlet end of the tank, and a "paddle wheel" mechanism may be provided to mechanically push the oil toward its collection site.
The gun-barrel separator has long experienced numerous problems. Oil tends to "weather" due to naturally occuring biodegradation and forms a basic-sediment-and-water layer, which is an emulsified water mixture commonly referred to as "BS & G layer" or "rag layer". This rag layer is highly elastic, and tends to form "clumps" which detract from the efficient separation of oil from the water. The paddle wheel tends to further emulsify the oil and contribute to the formation of a rag layer. Light oil with a density less than the rag layer cannot penetrate through the rag layer to the surface. The water tends to "channel" to the surface at the collection site, since the elastic rag layer resists uniform flow and "backs up" to the half-pipe. Consequently, water rather than oil is commonly recovered from the surface of the tank, and a high concentration of a entrained oil droplets flow with the water to the downstream system.
The disadvantages of the prior art are overcome by the present invention, and a relatively inexpensive and highly efficient system is hereinafter disclosed for separating a mixture of different density materials. The concepts of the present invention are particularly well suited for removing oil and other hydrocarbons from waste water.